Creating a panoramic, shaped backdrop with UV project modifier
I found out how to use an equirectangular image as a projected texture for projection mapping type scenarios. Maybe it's helpful to someone else, so i'll share it here.

Render an equirectangular panorama of the scene. This needs cycles and the camera set to panoramic instead of perspective.
Save the rendered image.
go to the thing you want to project on, go to the Shading-Environment and create a new Material.
add an &quot;Environment Texture&quot; Node and drop your Equirec-Render into it. Plug that into your Principled BSDF, or, if you don't want shadows and stuff, an Emission Node.
add a &quot;Texture Coordinate&quot; Node and connect it's object socket with the &quot;Environment Texture&quot;-Nodes vector socket.
add an &quot;Empty&quot; and place it at the specific location that is supposed to be the center of your projection. (in my case the place where a visitor would stand to see the diorama)
In the texture coordinate node, choose the empty for the &quot;Object&quot; field.
Tweak the projection by moving or rotating the empty.
Slab the material on anything you want to project onto.

The Node Setup. The projektor2 object is the empty defining the source or center of the projection


the diorama. The selected empty is the texture coordinate input